Microwave oscillator



Dec. 28, 1948.

Filed Jun 25, 1943 A. E. ANDERSON MICROWAVE OSCILLATOR l l i l hl l l hl -l l l l l l llhl l l 25 2 Sheets-Sheet 1 INVENTOR A. E. ANDERSON ATTORNEY Dec. 28, 1948. A. E. ANDERSON 2,457,194

MICROWAVE OSCILLATOR Filed June 2a, 1943 2 Sheets-Sheet 2 llllllilllilllLlllilll 64 63 D allllllllllllhhwlzls m/vE/v TOR A. E. ANDERSON BYLVM AT TORNE Y Patented Dec. 28, 1948 f 4 MICROWAVE OSCILLATOR V I Alva Eugene Anderson, Spring Lake, N. J.,. as

signer to Bell Telephone rated, New York, N. Y.,

Laboratories, Incorpoa corporation of New Application June 23, 1943, Serial No. 491,951

This invention relates to microwave oscillators ahd'more particularly to oscillation-producing systems utilizing electron velocity variation apparatus and resonant chambers for producing oscillations in the centimeter wavelength range.

Resonant chambers or cavities, as they are commonly termed, have many advantages for use in oscillators in the microwave art. However,

at extreme frequencies they become physicallyso'small that their structure is difficult to design and manufacture. Moreover, slight variations from the ideal result in comparatively large deviations in resonance frequency. The necessity ,for tuning appliances to adjust small reso-' nance cavities to the desired frequency increases these difficulties. Inasmuch as these conductive cavities or chambers are multi-frequency resonators. it is possible to resort to oscillations of one of the higher frequency modes. In this case the gravest frequency oscillations, that is, the lowest natural frequency mode oscillation which the conductive cavity will support, may. preponderate to 'such an extent as to dissipate most of the input energy and may cause heating and other undesirable interaction effects.

A principal object of the' invention is to enable an oscillator employing resonant cavities to operate at one of the higher modes free from the disturbance and loss attendant upon oscillation at a graver mode.

Another object of .the invention is to inhibit the operation of a cavity resonator at its gravest frequency while permitting it to operate freely at a frequency of a higher mode.

In accordance with the invention a microwave oscillator utilizing one or more resonant chambers is operated at a frequency higher than that of the gravest mode which any of the chambers will support. To avoid'the energy losses and other undesirable characteristics attending concomitant production of oscillations of the gravest mode such grave mode oscillations are inhibited. This may be effected by any of several expedients used singly or together.

One expedient for suppressing oscillations of the gravest mode relates to choice of the impelling eiectromotive force applied between the electron source and the collector of the principal electron'beam' If the magnitude of that electromotive force be so chosen that the transit time for the drift space between the .velocity variation gap and the energyderivation gap is correct fora' higher mode oscillation but is not correct for the gravest mode of oscillatiomoscillations of" the higher frequency will be sustained but 5 Claims. (cram-s) those of the gravest mode will be inhibited. In some instances it may be desirable to inhibit several of the graver modes of oscillation and by suitable chcice of the electromotive force this may be done. I

An alternative expedient is the use of a feedback path, thepath, beingso designed that its frequency transmission rangexpermits only oscillations of higher than the gravest mode or of higher than 'a few of the 'graver modes to freely pass.

lt' feature of'the invention is the use of a variable; magneticneldto focus the electron beam at a chosen point in its course. This is eflected by .a' permanent magnet structure which comprises the resonant chamber itself. Variable magnetic'shunts'enable the magnetic field strength to whi'chjthe beam is subjected to be varied as desired.

In the drawing Fig. 1 illustrates diagrammatically a microwave oscillator of the single resonance chanibertype in which oscillations of the gravest modeare inhibited by selection of the electron impelling electromotlve force.

Fig.1 illustrates in similar manner a. tworesonat'or oscillator with a wave guide feedback path which'iin'hlbits oscillations of the gravest mo'de.

Fig. aisle. modification of the disclosure of Fig. 2 in whichth'e' feedback comprises a plurality of internal wave guides.

Fig. 4 is a cross-sectionalong the line 4-4 of Fig. 3;

Fig. 5 shows a two-chamber oscillator in which selection of' a higher order mode of oscillation and inhibition of the gravest mode is effected by'choice of force.

Fig.'6 is-a modification of the disclosure of Fig. 3. in which the internal wave guides are replaced by, aperture or iris couplings.

Referring to Fig. 1, an electron discharge device il comprises an evacuated container I! of .dielectricmaterialenciosing an electron gun l3 of any suitable type 'anda cup-shaped electron collector ll which maybe of graphite or other appropriate materlaL' Sealed through the walls ofthe cont'ainer.'.'l2 are the conducting discs i8 and lQ-eachapertured at the cen er as indicated to permit the passage of a beam of electrons therethrpugli fromthe gun I. to the collector ll. Between the discs I! and II is a cylindrical conducting rnember .ll with end apertures aligned withith'e' aperture: in the discs I! and It to form so cailcd gaps ii and ii. The member ll prothe electron impelling electromotive vides a substantially field-free drift space of well-known type between the gaps l8 and I8. Surrounding the container I2 and constituting with the discs [5 and IS an enclosing resonant cavity is the two-part shell 20 consisting of permanent magnetic material. For this purpose any suitable permanent. magnetic material maybe used as. {on example the silos disclosedin. the patent to Jonas 2,295,082, September 8, 1942. The magnetic material of the cavity resonator, shell may be permanently magnetized with the central portion immediately adjacenhiha outer periphery of disc I! polarized as a positiuemag; netic pole and the central portion adlwimt. the outer periphery of disc l8 polarized amgnngatlya magnetic pole or with the reverse magnetization, as desired. The shell 20 may preferably be divided along a diametrical' plane passingthrough the longitudinal axis of container ii to permit ready assembly Clamps 2| serve to hold the-inwardlyproiectins. rims. 0L discs I! and I8. in. close encasement. with. the. suriace. oi. the shell ill-audio. close. any. wrtures. thumbs.- tween from an. electromagueticst ndncinti The. drift space member I? is SUPPOl'tBd-DZM8J11G. rods 22. eachattachedtothe. member. i]. at on Ill" end andanchoredatthecppositacndiimthe well oiltheinsulatinacontainer l2; Meadllpassinlthrough an. insulating head 25' in. the shell 21L serves. to. electrically. connect. themeniber I]. to an external circuit.

A source of electromotive. iorca 25, provided with. potentiometers Hamill may serve team the customary, polarizing potentials, to.the.- electron. gun. 11,. electron. collector l4. and. the. rose nantcavitycver loads 2). "land 3ll.,reaneotivels. As is. well understood. inthia art. abeam. oi. sled-- tronsproceed ns irnmthccunli. is subjected. at.

the gap l8'to the'electromagnetic ileld oi; the. resonantchamberi tocause the velocities at th passing, electrons tobavariodinaccordan o. with. the instantaneousmaanitudc of. thaoleotricileld atthat point... At the gap limiter transit across. the drift space. thaelectronawlll have. become.- bunched so. that the. stream is:- cbargo d nsity varied. The. density. varied. electron. stream. raracts, with. the electroma ne ic. field of; tho.- rclcrnant chamber to yield energy thereto in a manner. well-known. in this art. Accordlnalntheanparatus serves. to produce. oscillations, ol'. (mommie determined principally by thernonancecharaca teristics of. the. r sonance. chamhsn. mement produce sustained oscillations it is essential that. the groupsor. bunches oielectronsarrlvingptthe c p it reach it atsu htlm atm omthe. whole. they will deliver more.energy,-t .t a elactromas; nctic fleldthan they, abstract from. it. lttranspires, therefore, that with a liven electromotiveiorce impressed by. the. source ll between. the electron source l3. andthe CQ BM I. land w th given potentials impressed; unonjhe. discs. I 5. and. I6 and upon thedrittspacc membcr (hills transit time required'iorelectrons-moccasins; the. can It. t r shmembcr II to. the can. ilm -li such .as. to. bring the clectronatnthcjw. it, a. time favorable. for oscillations otione heuuennl but not tor oscillations. a. dii'icrcnt ireq'ucma. Applicant has found that wituaparticiilar ll paratus beginning witnan elcctliomojillajprcemff 500 volts, applied between source. I) and colltst r It d srauualiy incressi r. that. electromotiva force that at. 800 volts oscililtionaotifio. contiameters wavelength corresm lnr. to the Bravest. mode. oi oscillation oi" t ercsonant cavity were. obtained. Calculations showed. that. the. ciriift.

time between the velocity varying gap and the energy extraction gap was approximately 1.75 cycles. As the voltage was increased the oscillations disappeared until at a point in excess of 1000 volts oscillations of 10.25 centimeters corresponding to a higher order resonance mode of the resonant. cavity. were obtained. Further increase in voltage caused the oscillations 01 10.25 centimeters wavelength to be suppressed and at a -.point below 2500 volts oscillations of 7.65 centimeters wavelength were obtained. The invention', therefore, enables very short waves to be produced with av device of such physical dimenslums s.- to lend. itself readily to manufacture.

It is-highlpimportant in the generation of very short waves to be: able to make some variation in the frequency in order to compensate for variations in manufacture of apparatus and to permit a. system to be tuned to any desired standard. With apparatus of extremely small magnitude thisls almost. dlfllcult matter sinceany attempt to. vary. the dimensions or volume, of theresonant. chamber is. attended. with problems. oi variable. mechanismswhich must beaccommodated in ex;- rcmcly limited snaca. Moreover, any variation whatever in. thevolume of every small resonant chamber is likely to. make an enormous. absolute. frequency, hange. in the. resonance irequency. The. appar tus oi the. invention. overcomes this. dlillculty in the-tit. is. 01. sumcient. magn tude to enable. customary. tuning. appliances. to. be. utilized nd the. amount by which they change the. resonance f equencies. 01. the chamber. may readilybe. made comparatively small. In theenparatus of Fig.1 reliance for. tuning is .hadjprlmarily upon. one. or. more tunlns. p u s. 32. of well-known. im It has been explained. that. selection of a desired hig er resonance. frequency and s ppression oi. sra cr. mode oi: oscillation may be had. by the. p p hoice or imp llins. c tromotiveiorceap plied ct cen ele tron source I; and collector I14. otus assume thatthe me nv l ty o1. electrons in their transit time between gaps i8 and [a is. a function of the electromotiveiorce appliedto the collector H. Actually, the situation is by no means that simple although the principle is, not affected. In the absence 01 an alternating elec, tromagnetic held within the resonant chamber electrons leaving the source H are accelerated as they approach the disc i5 and; accultdlnsly, enter the. c n 8 with i y u stantially determined by the potential. Qt the disc [5. It the transit chamber ll" be maintained at. the. same steady potential as. the. disc l5, electrons. will contin cin their transit. across the. gap. it. through the (hilt, space. and. across the gap. is. with substantially constant velocity and without. further acceleration since disc it is at. the same. Potential as disc It. The electron velocity. under such rcumstanc s. would therefore be. determined chiefly by the Position-oi. the variable tan or the lead 3.0. on the potentiometer 2B. HoweYer..the. ransit. t me will. be. diflerentirom. that assumed. t the position or the tap 33 upon. potentiometer-ll be such as to. impress. uponthe .ciriitspacechamr. '1 B votential difleront. irom the-tot. thediscs it an 4.6., As. disclosed herein the potential of chamber. 1.1. is. somewht lower than. that. oi the discs. Q nsaq entllt, electrons in their tram J.

across gap 1.8 will exp ricncaa, reductioniinvelo itv and will. proceed. thereaitcr hrou h the unit. space. at. the reduced velocity until they reach the i can. I it across which. they. will. be. accelp eratcd up to. he velocity with which theyentercd sop.- h. 'lhlawillrhave theeflect of. iccreasinsq the transit time. Conversely, positioning the tap 33 t impress upon chamber l1 a potential higher than that of discs l and [5 will result in decreasing the transit time within the drift space.

As is well known the tendency of an electron beam after leaving the cathode or electron gun is to diverge. If this occurs to an undesirable extent a large portion of the beam may be so spread out as not to pass through the aligned apertures and thus to be intercepted before doing useful work at the gap l9. In order to prevent such a divergence, it is a customary practice in this art to apply a concentrating or collimating magnetic field. Preferably, the field should be of such strength as to enable substantially all of the electrons to traverse the gap IS. The reaction between electrons and the electromagnetic field in their transit across the gap is enhanced if the electron passes very close to the margins of the gap apertures or, in other words, to the perimeter oi. the circular openings at the extremities of the gaps.

A certain degree of focussing is desirable and may be obtained by use of a magnetic field. According to one feature of this invention, the magnetic field is produced by the shell 20 of the cavity itself which consists of permanently magnetized material. In order to vary the strength of the magnetic field applied across the gap, a plurality of external shunts comprising U-shaped strips 34 and 35 of soft magnetic iron material, the free ends of which closely engage the external walls of the shell 20 may be employed. As these are moved inwardly toward the dielectric container l2 they tend to divert the magnetic field from the gaps. As they are drawn outwardly they permit increase of that field.

The resonance characteristic of a closed chamber of conducting material depends to a considerable extent upon the nature and conductivity of its interior surface. In order to maintain energy loss in the chamber at as low a point as possible it is desirable that the interior surface have a very high conductivity. Moreover, in the case of a shell of magnetic material it is desirable to lnterpose a conducting screen between the internal electromagnetic field and the magnetic material in order to reduce hysteresis loss at the surface of the material. Accordingly, a coating or plating SI of highly conducting material such as copper or copper overplated with silver may be provided. The internal surface and a portion of the circular periphery of tuning plugs 32 may be likewise plated for the same purpose.

Fig. 2 discloses an oscillation-producing apparatus in which the electron source, the electron collector and the polarizing source are designated as in Fig. l. Positioned with their apertures in alignment with the electron beam passing from source I3 to collector l4 are two toroidal cavity resonators 36 and 31 physically connected by a tubular conducting member 38 which encloses the drift space between gaps 38 and 40. A plurality of tuning plugs 4i, 42, similar to plug 32 of Fig. 1 may be provided for each of the chambers 38 and 31. Connecting the chamber 31 to the chamber 36 and coupled to each of these chambers by predesigned aperture or iris openings 43 and 44 is a wave guide 45. The coupling irises and the wave guide are given such dimensions as to freely pass oscillations of a frequency corresponding to a desired higher order resonance of the chambers 38 and 31 but to suppress oscillations of the gravest resonance mode of these two chambers. Variable tap 48 enables the polarizing potential applied to the chambers and 31 and to the drift chamber 38 to be appropriately selected. In operation the electron beam passing gap 39 is subjected to a velocity variation after which, in the course of transit through the drift space of chamber 38, it becomes charge density varied so that at gap 40 it may react with the electromagnetic field within chamber 31 to yield energy thereto. Oscillations of a desired frequency higher than that of the gravest resonance mode are fed back by the wave guide which I precludes feedback of the graver mode oscillations. In this manner, sustained oscillations of the desired frequency are produced. Energy of the desired oscillation frequency may be withdrawn from the resonant output chamber 31 by an output coupling loop 48 and a coaxial output circuit 48 as in the case of Fig. 1.

Fig. 3 discloses a modification of the apparatus of Fig. 2 in which the velocity varying chamber 50 and the output energy chamber 5| are wholly constituted by a cylindrical shell 52 divided into two portions by a thick walled disc 53 of conduct ing material which supports at its center a drift space member 54. Each of the chambers is provided with a plurality of tuning plugs 55. A plurality of tubular openings 56 as indicated in Figs. 3 and 4 pass through the disc 53 and serve as wave guides to feed back energy from resonance chamber 5| to chamber 50. These wave guides may be designed in a manner similar to the wave guide 45 of Fig. 2 to freely permit passage of oscillations of a desired higher order resonance mode and to preclude feedback of oscillations of the gravest resonance mode of either chamber. It will be understood that, as in the case of Fig. 2, the two chambers may be tuned to have substantially the same resonance charac-- teristics by means of the tuning plugs 55.

Fig. 5 discloses an oscillating system in which two resonance chambers 51 and 58 may be substantially identical in design with the resonance chambers 36 and 31 of Fig. 2. The tuning plugs 59 are in this instance shown at the ends of these structures rather than at their outer peripheries as in Fig. 2. A coaxial feedback line 60 having terminal coupling coils 6i and 62 serves to feed back output energy from chamber 58 to velocity varying chamber 51. In this structure the coaxial line 60 may be designed to transmit oscillations of the entire gamut of resonance frequencies of the two resonance chambers. In order to selectively sustain oscillations of a frequency corresponding to a higher order of resonance mode of the chambers 51 and 58, and at the same time, inhibit oscillations of one or more of the gravest modes of oscillation, the variable contactor 63 of potentiometer 64 may be positioned in accordance with the principle presented in detail in the discussion of Fig. l to predetermine the transit time of electrons traversing the drift space within tube 38 between gaps 65 and 6.3 so that oscillations of the gravest mode will not be sustained but oscillations of the desired higher order frequency mode will be sustained.

Fig. 6, like Fig. 3, comprises a central metallic shell 52- divided intotwo resonance chambers 50 and 5| respectively. At the ends of the shell insulating container portions 51 and 68 enclose the electron gun i3 and the electron collector I4, respectively. In lieu of the thick-walled disc 53 of Fig. 3. a thin-walled disc 89 is utilized. In this disc are apertures or irises 10 which correspond in function to the wave guides 58 of Fig. 3. The manner of design of such irises or apermemos titres "to permit transmissionitherethrough ot os cillations 01 a: desired frequency' whilez at the: sunshine; precluding: transmission ofloscillations oi substantially lower frequency is well under--- stood in the art. Accordingly, the aperture 10 willbe' designedto transmit the d'esi'redhigher= order" resonance -frequency and to suppress-- transmis sion 'of'one or'more of the graver mode frequen cies; Eacl'r'ofthe' resonator chambers=disclosed no the" various figures oi the drawing 1 may have itsinterior coated with low'resistance material as in'the-case of Fig. 1-. Moreover eachoi theresonator chambers may'consist oi'permanently' magnetized magnetic material to'assist in colli mation'oi the electron beam:

The electron source l3 'whichhasbeen referred to-vari'ously'as a source ora' gun may beoiany' well-known type as mayal'so the'el'ectron' co1'-- iector M.

It will be apparent that the invention greatlysimplifies the" design of oscillators of the'velocity" variation type at extremely high frequencies; It wilialso be apparent that it enables a nice' control or adjustment of" the operating'frequency of such devices to be had.

What is claimed is:

1. An oscillator comprising an electron" beam. apparatus including a source of an electron beam, 2. cavity resonator having a gap adjacent the path of the beam to vary the electron velocities irr'the' beam, a second cavity resonator'havin'g'a' gap'adiacent the path of the beam at a point of density variation therein to receive energy from the'beam, said resonators having their gravest mode or'operation' at substantially'the same-irequency and means coupling the second resonator to'the first for feeding backenergy thereto, said. coupling means having a transmission frequency range which is limited to frequenciesconsiderably above the frequency of the gravest mode whereby oscillations of a higher mode are produced and oscillations of the gravest mode are inhibited.

2. A microwave source comprising an electron discharge device including an electron gun, an anode, a source of eiectromotive iorce connected therebetween, a pair oi cavity resonators'each having an aperture adjacent the path of'the beam oi electrons which proceeds from the gun to the anode whereby the electromotive force ot'the field oi the first of said resonators serves to vary the velocities of passing electrons and the electromotive force oi the second of said resonators reacts with the passing electron beam to derive energy therefrom, means coupling said resonators together, said coupling means having a transmission frequency range which lies entirely above the gravest frequency mode of either resonator whereby only oscillations 0! a higher mode may be produced and one of said resonators having means connected therewith ior tuning the resonator toenable it to operate at high efficiency at the same frequency as the other resonator.

3'. An electron discharge device comprisingtwo toroidal cavity resonators oi conducting'materiai,

each resonator having a central opening passing therethrough and a circular aperture through which an internal electromagnetic field may ex-- tend across the opening, means holding'said reso-- 8 natbrs inmed positiomwithrespect to eachother witin'their'central openings aligned, an electron gum and a collector electrode connected to the resonatorsto pass an'- electron beam through the aligned'centralapertures, aplurality otconducting tubes extendingbetween the resonators and' opening into-each: resonator whereby oscillation energy mayhe -transmitted from one resonator to the other; tiles-tubes:- each having: acritical ire- I quency slightly-."higher than the irequency'of the= gravest: mode ofeither resonator: whereby the transmission ofoscillation energy of the gravest mode or either resonator is precluded;

4. Am-electron' velocity variation: device com-- prising arsource 0t electrons, an electron collector and an electrical: resonant syi'stem connected thereto comprising an external' hollow cylinder oiconductingrmaterial, an internal hollow"- cylinder of conducting: material fixed in coaxial: position within:.the externalrcylind'er by an annular'walli ofrconducting material which divides the external: cylinder longitudinally into two cavity resonators; the: ends at both-the external andi the: internal cylinder: being apertured at points aligned witln the path: of the:electron beampassing front-the" eiectrom source to. the: electron collector; and: coupling. meansi between saidtwo cavity 'resona-- tors comprising an opening in: the annular wall having. acut-cit frequency above the frequency oi" the: gravest mode oi oscillation 0f: eithen cavity resonator but less than the: frequency 01 tha mode in whichzoscillation'is desiredz.v

5: An electrondischarge device: comprising electron gun; an. anode; .a source 05 Ieiectromotive: force connected therebetween,lmeansaforming two cavity resonators-z longitudinally spaced betweem said electron gun and said anodeeach' resonator. having. an aperture adjacent-thepath of'the beam of electrons whichproceeds Irom'the! gun to the.

a anode, couplingmeansbetween said resonators;

having a high -pass transmission: characteristic to-prevent-roscillation at= the-gravestv mode of; the? said' c ity resonators,-.. and 1 means: for reducing: thencross-sectional area: at the:beam or. electrons".- in the-region oitheaperturesr comprising; per--- manentmagnets-for producing: or iocussing: fieldin-= the regionoi! said--. apertures.

ALVA EUGENEANDERSON.

REFERENCES CITED The following'reierences are of record in the file oi'tliis' patent:

UNITED STATES PATENTS 2,40Q,68 Andersona "0613 225. 1946. 

